LazyStringSplit.cs

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
#nullable enable

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace NuGet.ProjectModel
{
    /// <summary>
    ///     Splits a string by a delimiter, producing substrings lazily during enumeration.
    ///     Skips empty items, behaving equivalently to <see cref="string.Split(char[])"/> with
    ///     <see cref="StringSplitOptions.RemoveEmptyEntries"/>.
    /// </summary>
    /// <remarks>
    ///     Unlike <see cref="string.Split(char[])"/> and overloads, <see cref="LazyStringSplit"/>
    ///     does not allocate an array for the return, and allocates strings on demand during
    ///     enumeration. A custom enumerator type is used so that the only allocations made are
    ///     the substrings themselves. We also avoid the large internal arrays assigned by the
    ///     methods on <see cref="string"/>.
    /// </remarks>
    internal readonly struct LazyStringSplit : IEnumerable<string>
    {
        private readonly string _input;
        private readonly char _delimiter;

        public LazyStringSplit(string input, char delimiter)
        {
            if (input is null)
            {
                throw new ArgumentNullException(nameof(input));
            }

            _input = input;
            _delimiter = delimiter;
        }

        public Enumerator GetEnumerator() => new(this);

        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();

        IEnumerator<string> IEnumerable<string>.GetEnumerator() => GetEnumerator();

        public IEnumerable<T> Select<T>(Func<string, T> func)
        {
            foreach (string value in this)
            {
                yield return func(value);
            }
        }

        public string First()
        {
            return FirstOrDefault() ?? throw new InvalidOperationException("Sequence is empty.");
        }

        public string? FirstOrDefault()
        {
            var enumerator = new Enumerator(this);
            return enumerator.MoveNext() ? enumerator.Current : null;
        }

        public struct Enumerator : IEnumerator<string>
        {
            private readonly string _input;
            private readonly char _delimiter;
            private int _index;

            internal Enumerator(in LazyStringSplit split)
            {
                _index = 0;
                _input = split._input;
                _delimiter = split._delimiter;
                Current = null!;
            }

            public string Current { get; private set; }

            public bool MoveNext()
            {
                while (_index != _input.Length)
                {
                    int delimiterIndex = _input.IndexOf(_delimiter, _index);

                    if (delimiterIndex == -1)
                    {
                        Current = _input.Substring(_index);
                        _index = _input.Length;
                        return true;
                    }

                    int length = delimiterIndex - _index;

                    if (length == 0)
                    {
                        _index++;
                        continue;
                    }

                    Current = _input.Substring(_index, length);
                    _index = delimiterIndex + 1;
                    return true;
                }

                return false;
            }

            object IEnumerator.Current => Current;

            void IEnumerator.Reset()
            {
                _index = 0;
                Current = null!;
            }

            void IDisposable.Dispose() { }
        }
    }

    internal static class LazyStringSplitExtensions
    {
        /// <remarks>
        ///     This extension method has special knowledge of the <see cref="LazyStringSplit"/> type and
        ///     can compute its result without allocation.
        /// </remarks>
        /// <inheritdoc cref="Enumerable.FirstOrDefault{TSource}(IEnumerable{TSource})"/>
        public static string? FirstOrDefault(this LazyStringSplit lazyStringSplit)
        {
            LazyStringSplit.Enumerator enumerator = lazyStringSplit.GetEnumerator();

            return enumerator.MoveNext()
                ? enumerator.Current
                : null;
        }
    }
}